The invention relates to a tool for counterboring, a clearance hole such as a drilled and tapped hole, for the head of a bolt as required to be recessed below the top plane of the part. The drilled hole is normally counterbored to a depth at least equal to the thickness of the head of the bolt or cap screw. Conventional tools are available for such application classified as counterbore cutters. Early art includes solid high speed cutters, also with carbide braised in place.
The present invention is more particularly directed to counterbore cutters employing indexable inserts. Such cutters are known in the art which include the use of small I.C. 60.degree. included triangles positioned to provide a plurality of cutting edges. In order to provide square corners at the bottom of the counterbore, the active cutting edge is positioned in a substantially radial line with a corner extending to the radius of the counterbore. Pockets for such inserts in the counterbore tool include two shoulders for engaging the sides of the insert for the inactive cutting edges. Conventional central holddown screws are offset to force the insert against such shoulders.
"Trigon" tool inserts are known in the art which are six sided, having three cutting corners, each with an 84.degree. included angle, as distinguished from a 60.degree. angle of a conventional triangle insert. As employed in drilling, the insert is positioned with the cutting point and center of the insert in a substantially axial line for the drill so that the active cutting edge extends at a positive radial rake angle of 6.degree..